signed_data: fix code block missing close marker

Author: cpu

src/signed_data.rs

@@ -92,6 +92,7 @@ impl<'a> SignedData<'a> {
     ///     signatureAlgorithm AlgorithmIdentifier,
     ///     signatureValue BIT STRING
     /// }
+    /// ```
     ///
     /// OCSP responses (RFC 6960) look like this:
     /// ```ASN.1